<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
</head>
<body>
    <ul>
        <li >html</li>
        <li>css</li>
        <li>javascript</li>
    </ul>
    <script>
        var lis=document.getElementsByTagName("li")
        for(var i=0;i<lis.length;i++){
            lis[i].onclick=function(){
                /* 1.点击时首先让所有的li变成黑色 */
                for(var j=0;j<lis.length;j++){
                    lis[j].style.color="#333"
                }
                /* 2.让当前项变红色 */
                this.style.color="red"
            }
        }
    </script>
</body>
</html>